The present invention generally relates to an apparatus and a method for drying semiconductor wafers in a solvent dryer and more particularly, relates to an apparatus and a method for preventing solvent droplets from falling on wafers during a solvent drying process in the solvent dryer.
In the fabrication of semiconductor devices, a large quantity of deionized (DI) water is frequently used to clean wafers in a wet bench process. For instance, when residual chemical must be removed from the surface of a wafer, DI water rinse is used in the wet bench process to perform major wafer cleaning operations such as quick-dump-rinse and cascade overflow rinse. It is desirable that the surface of the wafer be cleaned by DI water after a chemical or polishing process has been conducted on the wafer, i.e. oxide or nitride deposition, etching or chemical mechanical polishing process. The wet bench wafer cleaning step can be accomplished by equipment that is installed either in-line or in a batch-type process.
A typical automated wafer scrubber combines brush and solution scrubbing by DI water. The scrubber utilizes a hyperbolic high-pressure spray of DI water with a retractable cleaning brush. A typical wafer scrubbing process consists of a DI water spray step followed by a spin dry and nitrogen gas blow dry step. More recently, the solvent drying technology such as the use of isopropyl alcohol (IPA) has been developed to further improve the drying technology.
In a solvent drying technology, such as one that utilizes IPA shown in FIG. 1, the drying process is conducted in a static manner, i.e., with the wafer positioned statically without movement. The wafer dryer 10 is constructed of a drying tank 12 equipped with a wafer receptacle 14, a chiller 16, a sidewall heater 18 and a bottom heater 20. A cleaned and wet wafer is transported into the drying tank 12, or the vapor chamber. Vapor of IPA is transported into the chamber cavity 22 by a carrier gas such as a high purity nitrogen, or any other high purity inert gas. The vapor enters into cavity 22 is heated by the bottom heater 20 such that IPA is further vaporized and rises into the cavity 22.
The wafer 24 is surrounded by the IPA vapor and, due to the high volatility of IPA, water on the wafer surface can be evaporated away without leaving any water mark, contaminating particles or metal particles. The vapor pressure of IPA can be suitably adjusted such that there is a steady flow of IPA vapor in the cavity 22 fed from the IPA reservoir tank 26.
In the conventional IPA dryer 10 shown in FIG. 1, the only moving part for transferring wafers into and out of the chamber cavity is a robot arm. There are no other moving parts that can produce contaminating particles. The IPA drying chamber can thus be kept in an extremely clean condition to avoid any contamination of the wafer surface. To further maintain the cleanliness of the chamber cavity 22, an air filter 28 is utilized for filtering incoming air into the cavity 22 and for providing a suitable flow rate of the IPA vapor. After the cleaning process is completed, the water-containing IPA vapor is condensed by the chiller 16 into IPA liquid and is collected at the bottom of the drying chamber 12 for recycling and reuse by the process. The IPA vapor drying process is normally controlled by three major parameters, i.e. the purity and the water content of IPA; the flow rate and flow speed of the IPA vapor; and the cleanliness of the IPA vapor.
Another solvent drying technique has been developed in recent years which is similar in principal to that described. In a Maragoni dryer, the drying principal is based on the different surface tensions of IPA and DI water. The different surface tensions cause the ejection of water molecules from the wafer surface which are then collected by a reservoir in the drying apparatus. The Maragoni drying process is carried out by slowly withdrawing a wafer from a DI water tank immersed in DI water. At the same time, IPA vapor carried by N2 carrier gas is flown onto the wet wafer surface such that IPA is saturated on the exposed wafer surface above the water level. Since the concentration of IPA on the surface of the exposed wafer is larger than the concentration of DI water, the surface tension of IPA is smaller than the surface tension of water in the water tank. This causes water molecules on the surface of the exposed wafer to be retracted into the water tank and thus achieving the drying purpose.
A typical Maragoni dryer 40 is shown in FIG. 2. The Maragoni dryer 40 is constructed by an upper chamber section 52, a lower chamber section 44 which is also an outer tank, an inner tank 42 for holding a volume of DI water 62 therein, a drain conduit 50 in fluid communication with the outer chamber 44, a wafer carrier 46 for carrying a plurality of semiconductor wafers 60, an elevator means 48 for raising and lowering the wafer carrier 46 into and out of the volume of DI water 62, and a tank cover, or lid member 54. The outer tank 44 is formed by a tank wall 68 defining a cavity 56 therein for receiving an overflow of DI water 62 from the inner tank 42 when the wafer cassette 46 is lowered into the volume of DI water 62. The inner tank 42 is defined by sidewall 72 for holding the volume of DI water 62 therein. A cavity 58 is formed when the wafer carrier 46 is lowered into the volume of DI water 62 and the tank cover 54 is slid over the top of the inner tank 42 forming a hermetically sealed chamber.
In either the solvent dryer or the maragoni dryer, a chiller arrangement may be utilized to condensed, collect and recycle the solvent vapor. This is shown in FIG. 1 as chiller 16 which condenses the water-containing IPA vapor into IPA liquid such that it may be collected at the bottom of the drying chamber 12 for recycling and reuse. The chiller 16 is constructed of a plurality of condenser coils which are kept at a temperature below the vaporization temperature of the solvent. The plurality of condenser coils 70 are shown in FIG. 3. The solvent dryer 74 shown in FIG. 3 is similar to that shown in FIG. 1 with the chamber cavity shown in an enlarged view. A processing problem occurred when condensed solvent droplets 76 fall from the condenser coils 70 onto the surface of wafer 24 during the vapor drying process. The solvent droplets, when fall on the wafer surface, can cause severe contamination of the wafer.
It is therefore an object of the present invention to provide a solvent dryer for drying semiconductor wafers that does not have the drawbacks or shortcomings of the conventional solvent dryers.
It is another object of the present invention to provide a solvent dryer that is equipped with condenser coils and drip-proof guards for preventing solvent condensation from falling on the wafer.
It is a further object of the present invention to provide a solvent dryer for semiconductor wafers that is equipped with a plurality of condensing plates attached to a plurality of condenser coils.
It is another further object of the present invention to provide a solvent dryer for semiconductor wafers that is equipped with a plurality of condensing plates which are kept at a temperature below the vaporization temperature of the solvent.
It is still another object of the present invention to provide a method for preventing solvent droplets from falling on wafers during a solvent drying process for semiconductor wafers.
In accordance with the present invention, an apparatus and a method for preventing solvent droplets from falling on semiconductor wafers during a solvent drying process are provided.
In a preferred embodiment, a solvent dryer equipped with condenser coils and drip-proof guards is provided which includes a cavity for holding a wafer therein; means for introducing a solvent vapor in the cavity; a plurality of condenser coils positions on an inside wall of the cavity; and a plurality of condensing plates affixed to the plurality of condenser coils on a surface facing the wafer for condensing solvent vapor and flowing condensed solvent into a reservoir thus preventing solvent droplets from falling on the wafer.
In the solvent dryer that is equipped with condenser coils and drip-proof guards, the means for introducing a solvent vapor further includes a quantity of solvent that has a vapor pressure at least that of isopropyl alcohol stored at the bottom portion of the cavity; and heating means for heating the quantity of solvent to a temperature above a vaporization temperature of the solvent. The means for introducing a solvent vapor in the cavity may further include an exterior solvent vapor generating means and a conduit for flowing the solvent vapor into the cavity. The plurality of condenser coils may be arranged parallelly on the inside wall of the cavity. The plurality of condenser coils may be fabricated of a ceramic material, or may be fabricated of quartz. The plurality of condenser coils may further include a coolant flown therethrough for keeping the plurality of condenser coils at a temperature below the vaporization temperature of the solvent. The plurality of condensing plates may be formed of a ceramic material substantially similar to the material that forms the plurality of condenser coils, or the plurality of condensing plates may be formed of quartz. The plurality of condensing plates may be fused to the plurality of condenser coils. Each of the plurality of condensing plates may be fixed to the plurality of condenser coils in a direction perpendicular to the bottom of the solvent dryer, or may be fixed to the plurality of condenser coils in a tilted manner with a bottom of a condensing plate tilting away from the plurality of condenser coils. The solvent vapor may have a vapor pressure at 25xc2x0 C. at least that of isopropyl alcohol.
The present invention is further directed to a method for drying a wafer in a solvent dryer which can be carried out by the operating steps of providing a solvent dryer that is equipped with a cavity for holding a wafer therein, means for introducing a solvent vapor in the cavity, a plurality of condenser coils positioned on an inside wall of the cavity, and a plurality of condensing plates affixed to the plurality of condenser coils on a surface facing the wafer; positioning a wafer that has water molecules on its surface in the cavity; introducing a solvent vapor in the cavity and drying the wafer; and condensing solvent vapor that contains water molecules on surfaces of the plurality of condensing plates and collecting in a reservoir.
The method for drying a wafer in a solvent dryer may further include the step of forming the plurality of condenser coils and the plurality of condensing plates in a ceramic material. The method may further include the step of fusing each of the plurality of condensing plates to a plurality of condenser coils, or the step of flowing a coolant of not higher than 20xc2x0 C. through the plurality of condenser coils. The method may further include the step of cooling the plurality of condensing plates to a temperature of not higher than 20xc2x0 C., and preferably to a temperature of not higher than 17xc2x0 C. The method may further include the step of introducing a solvent vapor from a solvent that has a vapor pressure at 25xc2x0 C. at least that of isopropyl alcohol, or the step of introducing a solvent vapor from isopropyl alcohol.